[GIT PULL] close_range fix

[Christian Brauner]

Hi Linus,

/* Summary */
Syzbot reported a bug in close_range. Debugging this showed we didn't
recalculate the current maximum fd number for CLOSE_RANGE_UNSHARE |
CLOSE_RANGE_CLOEXEC after we unshared the file descriptors table.

So max_fd could exceed the current fdtable maximum causing us to set excessive
bits. As a concrete example, let's say the user requested everything from fd 4
to ~0UL to be closed and their current fdtable size is 256 with their highest
open fd being 4. With CLOSE_RANGE_UNSHARE the caller will end up with a new
fdtable which has room for 64 file descriptors since that is the lowest fdtable
size we accept. But now max_fd will still point to 255 and needs to be
adjusted. Fix this by retrieving the correct maximum fd value in
__range_cloexec().

I've carried this fix for a little while but since there was no linux-next
release over easter I waited until now.

With this change close_range() can be simplified a bit but imho we are in no
hurry to do that and so I'll defer this for the 5.13 merge window.

(Fwiw, the two follow-up patches sit in
 https://git.kernel.org/brauner/h/fs/close_range.)

/* Testing */
All patches have seen exposure in linux-next and are based on v5.12-rc4.
The selftests pass and the reproducer provided by syzbot did not trigger. The
patch also has a Tested-by from Dmitry but I had already pushed it out by the
time that came in so it's missing from the patch itself.

/* Conflicts */
At the time of creating this pr no merge conflicts were reported. A test merge
and build with today's master 2021-04-08 12:20:00 CET worked fine.

The following changes since commit 0d02ec6b3136c73c09e7859f0d0e4e2c4c07b49b:

  Linux 5.12-rc4 (2021-03-21 14:56:43 -0700)

are available in the Git repository at:

  git@xxxxxxxxxxxxxxxxxxx:pub/scm/linux/kernel/git/brauner/linux tags/for-linus-2021-04-08

for you to fetch changes up to 9b5b872215fe6d1ca6a1ef411f130bd58e269012:

  file: fix close_range() for unshare+cloexec (2021-04-02 14:11:10 +0200)

Please consider pulling these changes from the signed for-linus-2021-04-08 tag.

Thanks!
Christian

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for-linus-2021-04-08

----------------------------------------------------------------
Christian Brauner (1):
      file: fix close_range() for unshare+cloexec

 fs/file.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)